We all know that the conditions under which crystals are grown can affect their size and shape. But Guangnan Meng and colleagues at Harvard University in the US wanted to investigate the effects of elastic stress on them, which is increased by growing crystals on a curved surface rather than a flat one.

They tried to grow two-dimensional colloidal crystals on the surface of spherical water droplets and used confocal microscopy to watch what happened. Normally, when grown on a flat surface, the tiny crystallite particles merge together to form flat crystal sheets that grow bigger and bigger. But on a curved surface they can’t fit together, and this tension forces them into narrow, ribbon-like crystalline structures that make a branched pattern on the surface of the drop.  The team say their observations could be useful in some areas of nanotechnology, for example assembling viral capsids or phase separation on vesicles.